1. Field of the Invention
The present invention relates to the combustion of sulfur-containing fuels so that minimal emission of gaseous sulfur compounds occurs. It particularly relates to the substantially complete combustion of sulfur-containing carbonaceous and hydrocarbon fuels so that substantially reduced emission of gaseous sulfur compounds occurs.
2. Background Art
Within the past few years there has been an increasing concern with the immediate and long-term problems resulting from the ever-increasing pollution of the atmosphere. With this concern has come an awareness at all levels that steps must be taken to halt the increasing pollution and, if at all possible, to reduce the present pollution levels. As a result of this awareness, a substantial amount of money and effort is being spent by business and governmental agencies to develop standards and measures for preventing significant discharge of pollutants into the atmosphere. Among the pollutants of concern are the various oxides of nitrogen and gaseous sulfur compounds present in the waste gases discharged from many refining and chemical plants and the flue gases from power plants which generate electricity by combustion of fossil fuels. A predominant form of nitrogen oxide released to the atmosphere is nitric oxide (NO) which, upon release into the atmosphere, comes into contact with oxygen and can react therewith to form nitrogen dioxide (NO.sub.2) or any of the other numerous oxides of nitrogen, many of which are known to be toxic to both plant and animal life. The gaseous sulfur compounds may be present in many forms, such as H.sub.2 S, COS, SO.sub.2, and the like. These gaseous compounds are released into the atmosphere, come into contact with oxygen and moisture, and can react to form sulfuric acid, resulting in the so-called "acid rains", known to be detrimental to both aquatic and plant life.
Principally, three approaches have been utilized in attempts to reduce the emission of gaseous sulfur compounds. They are (1) removal of the sulfur constituents from the fuel prior to its combustion or partial combustion, (2) use of an additive to react with the sulfur during combustion, and (3) scrubbing of the gaseous effluent to remove the sulfur constituents prior to its release into the atmosphere. To obtain substantially complete removal of the sulfur constituents prior to combustion requires the use of expensive solvents for extraction of the sulfur components, which also extract a significant fraction of the fuel energy. Such methods have not proven to be altogether satisfactory in view of both cost and effectiveness. The predominant method now practiced for removal of the sulfur constituents comprises scrubbing the effluent gases with an absorbent for removal of the sulfur constituents prior to discharging the gases into the atmosphere. A disadvantage with this approach, however, is the high capital and operating costs involved in treating the large volumes of effluent gas to remove the small quantity of dispersed gaseous sulfur components. Indeed, for an average utility power plant, the cost of a facility for treating the effluent gas can be in excess of $100 million.
Much effort has been directed to attempting to react the sulfur compounds present in the fuel with an additive to form solid compounds during the combustion process. Advantages of this approach are that: (1) the absorbent can be placed in more intimate contact with the fuel sulfur compounds and, therefore, the absorbent treats a higher concentration of these sulfur compounds; (2) the capture of the fuel sulfur in a solid, easily removable form is accomplished in existing equipment, the burner; and (3) the solid sulfur compounds can then be removed by existing filtration equipment, which is normally used for the removal of the ash constituents of the fuel. The principal disadvantage of this approach heretofore has been that excessive amounts of absorbent had to be added to the fuel to obtain a high percentage of removal of the sulfur constituents of the fuel. Indeed, most of the literature suggests that a molar ratio of absorbent to sulfur of at least 3 or higher is necessary to remove 70% or more of the sulfur.
Much effort has also been directed to retaining the solid sulfur compounds as solids throughout the remainder of the combustion process. The literature indicates that, although gaseous sulfur compounds are thermodynamically favored over the solid forms at normal combustion stoichiometry and temperatures, nearly all experiments show that some fraction of the solid forms survive the combustion process. (See G. Flament, "Direct Sulphur Capture in Flames Through the Injection of Sorbents", Int'l Flame Res. Fdtn. Doc.nr. G 19/a/9, Nov. 1980.) In most cases, about half of the fuel sulfur survives the entire combustion process in the solid form, and some experiments have shown considerably higher retention.
Obviously, there still exists a need for an improved method and apparatus for the combustion of sulfur-containing fuels which would permit a reduction in excess of 70% and preferably in excess of 90% of the gaseous sulfur compounds which would otherwise be emitted and which would not require the use of a large excess of absorbent.